1. Field of the Invention
The invention relates to an exhaust gas after-treatment system for an internal combustion engine, in particular for a marine diesel engine operated with heavy fuel oil. The invention furthermore relates to a method for the exhaust gas after-treatment of exhaust gas leaving an internal combustion engine, in particular a marine diesel engine operated with heavy fuel oil.
2. Description of the Prior Art
In the case of internal combustion engines operated with heavy fuel oil there is the possibility that the fuel used, i.e. the heavy fuel oil, has a high sulphur content. Sulphur oxides can react with other components of the exhaust gas and result in deposits that impair the effectiveness of the exhaust gas emission control. This is disadvantageous. There is a need for an exhaust gas after-treatment system for an internal combustion engine, which makes possible effective exhaust gas emission control even in the case of internal combustion engines operated with heavy fuel oil.
From DE 10 2004 027 593 A1 an internal combustion engine with exhaust gas turbocharging and exhaust gas emission control is known. The exhaust gas turbocharging is embodied either as single-stage or as two-stage exhaust gas turbocharging with an exhaust gas turbocharger or with two exhaust gas turbochargers. The exhaust gas emission control comprises an SCR catalytic converter, which in the case of the single-stage exhaust gas turbocharging, is positioned, seen in flow direction of the exhaust gas, either downstream of the turbine of the exhaust gas turbocharger or upstream of the turbine of the exhaust gas turbocharger. In the case of a two-stage exhaust gas turbocharging with a high-pressure exhaust gas turbocharger and a low-pressure exhaust gas turbocharger, the SCR catalytic converter is positioned between the high-pressure turbine of the high-pressure exhaust gas turbocharger and the low-pressure turbine of the low-pressure exhaust gas turbocharger.
An SCR catalytic converter of an exhaust gas after-treatment system uses ammonia as reduction agent for denitrifying the exhaust gases. In the case of exhaust gas after-treatment systems known from the prior art, ammonia is introduced into the exhaust gas upstream of the SCR catalytic converter either directly or an ammonia precursor substance, such as a watery urea solution, which in the exhaust gas flow is decomposed or evaporated to form steam, carbon dioxide, and ammonia. Here, according to practice, the quantity of the ammonia or of the ammonia precursor substance introduced into the exhaust gas upstream of the SCR catalytic converter is adjusted so that this quantity corresponds to the ammonia quantity that is convertible in the SCR catalytic converter, so that downstream of the SCR catalytic converter no ammonia is contained in the exhaust gas to avoid undesirable ammonia emissions.